1. Field of the Invention
The present invention relates to a constant voltage source with an output current control element, which adjusts an output voltage (u_out) of the constant voltage source by varying its output current, with a control circuit, which determines an actual value of the output voltage and processes it to a control signal for controlling the output current control element, and with a limiting circuit, which limits the output current to a predefined maximum value by action on the control signal. Furthermore, the invention relates to a method for generating a load-independent constant output voltage with such a constant voltage source.
2. Description of the Background Art
A constant voltage source is known from the publication “Halbleiterschaltungstechnik” (Semiconductor Circuit Technology) by Tietze and Schenk, ISBN 3-540-19475-4, 9th edition, Springer-Verlag Berlin Heidelberg New York, pages 544, 545, there in particular FIG. 18.11.
This figure shows an integrated voltage regulator with a power transistor, which outputs a variable output current, to provide a constant voltage. A first subcircuit, which has a resistive voltage divider and a differential amplifier, compares an actual value for the voltage with a reference value. An output signal of the differential amplifier represents a measure for the indicated deviation and is used to control the power transistor in a first feedback loop.
To limit the output current, the prior-art circuit has a current shunt in the output current branch and a second transistor, which in the conductive state reduces a control voltage of the power transistor. An excessively large voltage drop across the current shunt, which is generated by a too high output current, activates the second transistor and thereby reduces the control signal of the power transistor. The output current limitation in the prior-art circuit therefore occurs in a second feedback loop. The prior-art circuit has an undesirable oscillation tendency and must therefore be stabilized by additional measures.
WO 01/46768 A1, which corresponds to U.S. Pat. No. 6,407,537 also discloses a constant voltage source.
A disadvantage of the prior art is that due to the effect of the channel length modulation, the maximum value of the output current depends on the input or supply voltage. A further disadvantage is that the prior-art constant voltage source does not permit a higher input or supply voltage, because in this case a transistor would break down.